Viscous Clutch and Method of Operation

ABSTRACT

A viscous clutch includes a shaft, a rotor disk, a housing having a base, a working chamber, a reservoir fluidically connected to the working chamber, a valve, an electromagnetic coil, and a flux guide that passes through the housing. The rotor disk includes a conductive portion made of a magnetic flux conductive material that forms a hub of the rotor disk that contacts the shaft and another portion. The base includes a hub with an axially-extending ring, and an armature of the valve is located radially outward of the axially-extending ring. The electromagnetic coil is located at an opposite side of the rotor disk from the reservoir. A magnetic flux circuit extends from the electromagnetic coil to the flux guide, then to the armature of the valve, then to the conductive portion of the rotor disk, then to the shaft, and then back to the electromagnetic coil.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of International Application No.PCT/US2017/031868, filed May 10, 2017, and claims priority to U.S.Provisional Application Ser. No. 62/356,101, the contents of each ofwhich are hereby incorporated by reference in their entireties.

FIELD

The present invention relates generally to clutches, and moreparticularly to viscous clutches and components suitable for use withviscous clutches and methods of using the same.

BACKGROUND

Clutches (also called drives or couplings) are used in a variety ofcontexts to selectively control torque transmission between an input andan output. For instance, fan clutches are used to control rotation of afan, such as a cooling fan for an automotive or industrial application.Controlled operation of a cooling fan provides all the benefitsassociated with cooling flows when the clutch is engaged, but alsoallows the fan to be turned off when not needed, thereby reducingparasitic losses and increasing fuel efficiency. Turning off a coolingfan can also allow additional power to be diverted to other uses.

In certain light duty applications, such as for small industrialequipment like miniature excavators, generators, light towers, andmaterial handling equipment, there are severe space constraintsavailable that limit the use of a fan clutch. Many of these applicationsdo not include any fan clutch and currently use a fan that is directlydriven by a pulley on the engine. These types of “always on” cooling fanarrangements are compact, but are far from optimal in terms of fuel burnand parasitic losses. The space to introduce a fan drive was never takeninto account in the design. Furthermore, many of these applicationsutilize a fan that is mounted on the water pump, primarily so that thesame belt drive system can be used to power both the water flow and thefan, saving cost and complexity. However, many of the water pumps weredesigned to support the mass of the fan only. Increased weightintroduced by a fan clutch may require larger bearings in the waterpump, increasing size and cost of the water pump. Moreover, because thefan (and fan clutch) is hung in front of the water pump bearing system,the length is an equally large factor in the amount of overhung loadneed to be carried by the bearing system.

While some relatively small bimetal-controlled viscous fan clutches areavailable, many industrial applications utilize blower fans rather thansucker fans, meaning that relatively hot air is blown out from an enginecompartment rather than relatively cool air being sucked into the enginecompartment. Bimetal controlled viscous clutches are not suitable forsuch blower applications, because the heat from the engine compartmentair would tend to keep the clutch engaged all or almost all of the time.

Therefore, it is desired to provide a controllable fan drive that has acompact size suitable for light duty applications. Furthermore, or inthe alternative, it is desired to provide an electromagnetic coilassembly suitable for suit in a compact viscous clutch.

SUMMARY

In one aspect, a viscous clutch includes an input member, an outputmember, a working chamber defined between the input member and theoutput member, a reservoir to hold a supply of a shear fluid, a valveconfigured to control a flow of the shear fluid between the reservoirand the working chamber along a fluid circuit that fluidically connectsthe reservoir and the working chamber, a bearing, and an electromagneticcoil supported by the bearing. The electromagnetic coil includes a coilhousing and a winding that forms multiple turns within an interiorvolume of the coil housing, wherein the coil housing has a steppedconfiguration to at least partially accommodate the bearing within afirst step. Selective energization of the electromagnetic coil actuatesthe valve.

In another aspect, a method includes winding a wire within an interiorvolume of an electromagnetic coil housing to provide a coil, andengaging a bearing with the electromagnetic coil housing at the step.The wire makes multiple turns within the interior volume so as to span aradially outer volume and an adjoining radially inner volume. Theradially outer volume is located radially outward of a step formed by awall of the electromagnetic coil housing.

In another aspect, an electromagnetic coil assembly for use with aclutch includes a bearing that has inner and outer races and a pluralityof rolling elements positioned between the inner and outer races, awinding, and a coil housing defined by a wall supported on the outerrace of the bearing. The coil housing further defines an interior volumein which a plurality of turns of the winding are located, the turns ofthe winding located opposite the bearing across the wall. The interiorvolume includes a first portion having a first axial depth and a secondportion having a second axial depth. The first axial depth is greaterthan the second axial depth. The first portion is located radiallyoutward of the outer race of the bearing and the second portion extendsradially inward of the outer race of the bearing.

In another aspect, a viscous clutch includes a shaft, a rotor diskrotationally affixed to the shaft to rotate at all times with the shaft,a housing having a base and a cover, a working chamber defined betweenthe rotor disk and the housing, a reservoir to hold a supply of a shearfluid, the reservoir fluidically connected to the working chamber by afluid circuit and carried by the rotor disk, a valve configured tocontrol a flow of the shear fluid between the reservoir and the workingchamber along the fluid circuit, an electromagnetic coil that can beselectively energized to control actuation of the valve, and a fluxguide that passes through the housing. The rotor disk includes aconductive portion made of a magnetic flux conductive material andanother portion made of a different material. The conductive portionforms a hub of the rotor disk that contacts the shaft. The base includesa hub with an axially-extending ring, and an armature of the valve islocated radially outward of the axially-extending ring. Theelectromagnetic coil is located at an opposite side of the rotor diskfrom the reservoir. A magnetic flux circuit extends from theelectromagnetic coil to the flux guide, then to the armature of thevalve, then to the conductive portion of the rotor disk, then to theshaft, and then back to the electromagnetic coil. A torque couplingbetween the rotor disk and the housing is selectively provided as afunction of a volume of the shear fluid present in the working chamber.

In another aspect, a method of operating a viscous clutch includesenergizing an electromagnetic coil to generate magnetic flux, passingthe magnetic flux from the electromagnetic coil to a flux guide in ahousing of the viscous clutch across a first air gap, passing themagnetic flux from the first flux guide to an armature of a valve acrossa second air gap, passing the magnetic flux from the armature of a valveto a conductive hub portion of a rotor to across a third air gap,actuating the valve to control flow of a shear fluid within the viscousclutch as a function of movement of the armature, passing the magneticflux from the conductive hub portion of the rotor through a live shaft,and passing the magnetic flux from the live shaft to the electromagneticcoil across a fourth air gap.

In another aspect, a viscous clutch includes a shaft, a rotor diskrotationally affixed to the shaft to rotate at all times with the shaft,a housing, a working chamber defined between the rotor disk and thehousing, a reservoir to hold a supply of the shear fluid and fluidicallyconnected to the working chamber by a fluid circuit, a valve configuredto control a flow of the shear fluid between the reservoir and theworking chamber along the fluid circuit, an electromagnetic coil, afirst flux guide that passes through the housing, and a second fluxguide that passes through the housing and is located outward of thefirst flux guide. The rotor disk includes a conductive portion made of amagnetic flux conductive material and another portion made of adifferent material. The valve includes an armature. The electromagneticcoil is located at an opposite side of the rotor disk from thereservoir. The reservoir is carried by the rotor disk. A magnetic fluxcircuit extends from the electromagnetic coil to the first flux guide,then to the conductive portion of the rotor disk, then to the armatureof the valve, then to the second flux guide, and then back to theelectromagnetic coil. A torque coupling between the rotor disk and thehousing is selectively provided as a function of a volume of a shearfluid present in the working chamber. Selective energization of the coilcontrols actuation of the valve.

In yet another aspect, a method of operating a viscous clutch includesenergizing an electromagnetic coil to generate magnetic flux, passingthe magnetic flux from the electromagnetic coil to a first flux guide ina housing of the viscous clutch across a first air gap, passing themagnetic flux from the first flux guide to a conductive portion of arotor across a second air gap, passing the magnetic flux from theconductive portion of the rotor to an armature of a valve across a thirdair gap, actuating the valve to controls flow of a shear fluid withinthe viscous clutch as a function of movement of the armature, passingthe magnetic flux from the armature to a second flux guide in thehousing across a fourth air gap, and passing the magnetic flux from thesecond flux guide to the electromagnetic coil across a fifth air gap.

The present summary is provided only by way of example, and notlimitation. Other aspects of the present invention will be appreciatedin view of the entirety of the present disclosure, including the entiretext, claims and accompanying figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of an embodiment of a viscous clutchaccording to the present invention.

FIG. 2 is a cross-sectional view of the clutch, taken along line 2-2 ofFIG. 1.

FIG. 3 is a cross-sectional view of a portion of the clutch.

FIG. 4 is a cross-sectional view a portion of an electromagnetic coilassembly of the clutch, shown only above a central axis.

FIG. 5 is a cross-sectional perspective view of a rotor, reservoir andvalve assembly of the clutch, shown in isolation.

FIG. 6 is a cross-sectional view of another embodiment of a viscousclutch according to the present invention.

FIG. 7 is a cross-sectional view a portion the clutch of FIG. 6.

While the above-identified figures set forth embodiments of the presentinvention, other embodiments are also contemplated, as noted in thediscussion. In all cases, this disclosure presents the invention by wayof representation and not limitation. It should be understood thatnumerous other modifications and embodiments can be devised by thoseskilled in the art, which fall within the scope and spirit of theprinciples of the invention. The figures may not be drawn to scale, andapplications and embodiments of the present invention may includefeatures, steps and/or components not specifically shown in thedrawings.

DETAILED DESCRIPTION

In general, present invention provides an electronically controlledviscous clutch, suitable for use as a fan clutch, with anelectromagnetic coil on an engine side of the clutch, in which theclutch is extremely narrow (i.e., short in axial length) in order toreduce an overhung load supported by an associated bearing system (e.g.,a bearing system for a water pump that provides a torque output to theclutch). It has been found that a clutch with a relatively short axiallength can promote a compact clutch design with relatively low mass, andto facilitate reduced overhung loading. Although numerous features andbenefits of the invention will be recognized in view of the entirety ofthe present disclosure, including the accompanying figures, a number offeatures of the clutch help minimize the axial length, including: astepped electromagnetic coil, a magnetic flux circuit that passesthrough an input rotor disk to actuate a valve armature located betweenthe electromagnetic coil and the rotor disk, and a shaft that passesentirely through a housing of the clutch.

FIG. 1 is a perspective view of an embodiment of a viscous clutch 20,and FIG. 2 is a cross-sectional view of the clutch 20 taken along line2-2 of FIG. 1. As shown in FIGS. 1 and 2, the clutch 20 includes a shaft22, a housing 24, a rotor 26, a reservoir 28, a working chamber 30, avalve (or valve assembly) 32, and an electromagnetic coil 34. An axis Ais illustrated that defines a rotational axis of the clutch 20. Itshould be noted that the illustrated embodiment is disclosed merely byway of example and not limitation, and numerous alternative embodimentsare possible, some features of which are described in the text thatfollows.

The shaft 22 is located at a center of the clutch 20, and extendsthrough an entire axial length of the clutch 20 including the housing24. The shaft 22 can act as a primary structural support for the entireclutch 20, which is to say that mass of the clutch 20 can be supportedprimarily by the shaft 22, and in the illustrated embodiment the mass ofthe clutch 20 is essentially entirely carried by the shaft 22. As shownin the embodiment of FIG. 1, the shaft 22 includes a first engagementfeature 22-1 at a first or driving end 22D (also called the rear end)and a second engagement feature 22-2 at an opposite second or front (ordriven) end 22F. The shaft 22 can further have a stepped configuration,with a larger diameter at the driving end 22D and incrementallydecreasing to a smaller diameter at the front end 22F. The shaft 22 inthe illustrated embodiment is a “live” or driven input shaft, meaningthat the shaft 22 accepts a torque input to the clutch 20 and rotateswhenever the torque input is present. The shaft 22 then distributes thetorque input to other components of the clutch.

The first engagement feature 22-1 allows engagement with an externalprime mover (not shown), such as an engine drive shaft or a shaft of awater pump, that provides the torque input. In one embodiment, the firstengagement feature 22-1 is configured as an internal (female) thread,though in alternate embodiments other engagement mechanisms can be used(e.g., a flange for a bolted connection, a sheave/pulley, etc.). Outwardof the internal first engagement feature 22-1 can be a small axial stop22-3 and a cylindrical outer surface 22-4.

The second engagement feature 22-2 allows suitable tooling to engage theshaft 22 to attach or remove the entire clutch 20 from the prime mover.For instance, second engagement feature 22-2 can be an Allen headsocket, or other suitable type of socket (e.g., Reynolds, Torx®, etc.).Configuration of the second engagement feature 22-2 as an internal(i.e., female) structure helps to reduce the overall axial length of theclutch 20, though other configurations are possible in alternateembodiments, as discussed further below.

The presence of the second engagement feature 22-2 at the front end 22Fof the shaft 22 helps eliminate the need for an external engagementfeature such as a flange or a hexagonal tooling-accepting feature (i.e.,wrench flats) at or near the driving end 22D, and allows the driving end22D of the shaft 22 to solely internally engage a driven component ofthe prime mover. Such an internal engagement at the driving end 22D ofthe shaft 22 helps to shorten the overall axial length of the clutch 20,without appreciably increasing radial dimensions of the clutch 20. Axialclutch length reduction is possible, in part, because surroundingcomponents such as the electromagnetic coil 34 can axially overlap thedriving end 22D of the shaft 22 having an internal first engagementfeatures 22-1 more extensively than an external engagement feature wouldnormally permit, due to the need to maintain tool access to such anexternal engagement feature. Moreover, the front end 22F of the shaftwill typically be more conveniently accessible than the driving end 22Din many applications, such as in fan clutch applications in which fanblades secured to the clutch 20 restrict access to the driving end 22Dwithin the confines of an engine compartment.

The housing 24 includes a base 24-1 and a cover 24-2 secured to eachother in a rotationally fixed manner. The housing 24 is rotationallysupported on the shaft 22, allowing the housing 24 to selectively rotaterelative to the shaft 22 as a function of the operational state of theclutch 20 (i.e., from approximately 0% to approximately 100% of therotational speed of the shaft 22). In this respect, where the shaft 22(and rotor 26) accepts a torque input to the clutch 20, the housing 24can act as an output or output member of the clutch 20. Cooling fins24-3 can be provided on external surfaces of the housing 24, on the base24-1 and/or the cover 24-2, to facilitate heat dissipation to ambientair.

In the illustrated embodiment, the base 24-1 is supported on the shaft22 by a rear bearing 40 and the cover 24-2 is supported on the shaft 22by a front bearing 42. The rear and front bearings 40 and 42 are locatedon opposite sides of the rotor 26, and are axially spaced apart fromeach other. The bearings 40 and 42 are each single-row ball bearings inthe illustrated embodiment, but could have other configurations inalternate embodiments. Many prior art viscous clutches instead have asingle bearing set on the driving (engine) side of the clutch in orderto support both the clutch mass and the mass of an output device (e.g.,fan), which can be either a single or double row bearing depending onthe application. In such single bearing prior art clutches, a single-rowbearing can be problematic because there is less load capability and thepotential for deflection around the single row is relatively large whencompare to a double-row bearing. Yet in such prior art clutches adouble-row bearing occupies a relatively large axial length in a crucialaxially central portion of the clutch, which tends to increase theoverall axial length of the clutch undesirably. Use of two single-rowbearings 40 and 42 has three primary benefits. First, the presence oftwo spaced apart bearings 40 and 42 allows for relatively high stabilityand low deflection when compared to one single-row bearing. Second, theadditional front bearing 42 can be placed into the cover 24-2 of thehousing 24 in a space at or near the front end 22F of the shaft 22 thatis generally unused, which does not significantly add to the overallaxial length of the clutch 20. Finally, the front bearing 42 in thecover 24-2 provides a sealed opening in the cover 24-2 for the shaft 22.This enables the addition of the second engagement feature 22-2 on theshaft 22, accessible from the front side of the clutch 20, forinstallation and removal of the entire clutch 20 relative to an engineor water pump, for instance.

The housing 24 can be made of aluminum or another suitable material ormaterials. The cover 24-2 can be a die cast part, which has the benefitof being a single part and keeping a viscous shear fluid (e.g., siliconeoil) in the clutch 20 and dirt and debris out of the clutch 20. Asdiscussed below, inserts can be provided in portions of the housing 24(e.g., in the base 24-1) for magnetic flux conduction. One or more seals(not shown) can be provided along the housing 24 to further help retainshear fluid within the clutch 20, although in the illustrated embodimentthe front and rear bearings 42 and 40 seal the shear fluid inside thehousing 24 without the need for additional dedicated seal elements.Furthermore, an output device (not shown) such as a fan can be securedto the housing 24 to accept a torque output from the clutch 20.

The rotor 26 is positioned at least partially within the housing 24, andpreferably entirely within the housing 24, and can have a disk-likeshape (accordingly, the rotor 26 can be called a rotor disk). The rotor26 is rotationally fixed to the shaft 22, and rotates at all times withthe shaft 22. In this respect, where the shaft 22 accepts a torqueinput, the rotor 26 can function as the input or input member of theclutch 20.

In the illustrated embodiment, the rotor 26 has a conductive portion26-1 and a non-conductive portion 26-2, where “conductive” in thisinstance refers to magnetic flux conductivity. The conductive portion26-1 can be made of steel or another suitable magnetic flux-conducingmaterial (e.g., any ferromagnetic material), and the non-conductiveportion 26-2 can be made of aluminum or another suitable material thatdoes not readily conduct magnetic flux. The conductive portion 26-1 canbe configured as a radially inner hub of the rotor 26 that can bedirectly secured to the shaft 22 and is spaced from and separated fromthe reservoir 28. Moreover, in the illustrated embodiment the conductiveportion 26-1 abuts both the rear and front bearings 40 and 42. Thenon-conductive portion 26-2 can be located at a radially outer part ofthe rotor 26. In the illustrated embodiment, the conductive portion 26-1includes an axial offset region that includes a cylindrical portion withan inward-facing cylindrical surface 26-3, as well as an exposed axiallyrearward facing portion 26-4. Furthermore, in the illustratedembodiment, the conductive portion 26-1 forms the entirety of the rotor26 over a given radial extent (e.g., to or beyond the cylindricalsurface 26-3), and, in that respect, is a structural component that isdistinguished from a non-structural flux guide insert that is embeddedin or otherwise passes through surrounding structural material of arotor solely to conduct magnetic flux. The conductive portion 26-1 canalso extend radially outward from the shaft 22 over a significantdistance, making the conductive portion 26-1 more than a mere hub orinner sleeve. As shown in the illustrated embodiment, the conductiveportion 26-1 extends beyond the axial offset region and the cylindricalsurface 26-3, but is located radially inward of the working chamber 30.Moreover, in the illustrated embodiment, the conductive portion 26-1extends from the shaft 22 to a location at or radially outward from ahub of the base 24-1 of the housing 24 (and/or an inner hub/second fluxguide, discussed below) as well as outward of the rear bearing 40.

The reservoir 28 provides a storage volume to hold a supply of the shearfluid. In the illustrated embodiment, the reservoir 28 is provided on orwithin the rotor 26. A plate 28-1 of the reservoir 28 can be attached toand carried by the rotor 26 to form part of a boundary to help retainthe shear fluid and to separate the reservoir 28 from other portions ofthe clutch 20. The plate 28-1 can be located in an interior of theclutch 20, and can be arranged at a front side of the rotor 26 thatfaces the cover 24-2. All or part of the shear fluid can be stored inthe reservoir 28 when not needed for engagement of the clutch 20. In theillustrated embodiment, the reservoir 28 is carried by the rotor 26,such that the reservoir 28 and shear fluid contained within both rotatewith the rotor 26. In this way, when the shaft 22 and the rotor 26 actas the input to the clutch 20, the reservoir 28 rotates at input speedwhenever there is a torque input to the clutch 20, which imparts kineticenergy to the shear fluid in the rotor-carried reservoir 28 tofacilitate relatively quick clutch engagement response times. An outletbore 44 is provided along the boundary of the reservoir 28 to allow theshear fluid to pass to the working chamber 30 along a fluid circuit ofthe clutch 20. In the illustrated embodiment, the rotor 26 forms part ofthe boundary of the reservoir 28, and the outlet bore 44 passes throughthe rotor 26. More specifically, in the illustrated embodiment theoutlet bore 44 passes substantially axially through the non-conductiveportion 26-2 of the rotor 26 at a location outward from the conductiveportion 26-1.

The working chamber 30 is defined (and operatively positioned) betweenthe rotor 26 and the housing 24. The working chamber 30 can extend toboth sides of the rotor 26. As explained further below, selectiveintroduction of the shear fluid (e.g., silicone oil) to the workingchamber 30 can engage the clutch 20 by creating a viscous shear couplingto transmit torque between the rotor 26 and the housing 24, with thedegree of torque transmission (and associated output speed) beingvariable as a function of the volume of shear fluid present in theworking chamber 30. Concentric annular ribs, grooves and/or othersuitable structures can be provided on the rotor 26 and housing 24 toincrease surface area along the working chamber 30 and promote a shearcoupling when the shear fluid is present in the working chamber 30, asis known in the art. Moreover, openings (not shown) can be provided inan outer diameter region of the rotor 26 to allow the shear fluid in theworking chamber 30 to move between opposite side of the rotor 26, in amanner well-known in the art.

The shear fluid is pumped from the working chamber 30 back to thereservoir 28 along a return bore 46, which is located in the housing 24in the illustrated embodiment. The pumping of the shear fluid into thereturn bore 46 can occur continuously using a dam or baffle (notspecifically shown), as is known in the art. Such a dam can be locatedon the housing 24 adjacent to the inlet of the return bore 46. The fluidcircuit of the clutch 20 therefore extends from the reservoir 28 to theworking chamber 30 through the outlet bore 44, and then from the workingchamber 30 back to the reservoir 28 through the return bore 46.

The valve 32 selectively controls flow of the shear fluid between thereservoir 28 and the working chamber 30. The clutch 20 can beelectromagnetically controlled, meaning that selective energization ofthe electromagnetic coil 34 can control operation of the valve 32 inorder to control the volume of the shear fluid present in the workingchamber 30, and in turn the degree of engagement and torque transmissionbetween the input and output members. In the illustrated embodiment, allmoving parts of the valve 32 are contained within the housing 24, andthe valve 32 is positioned in between the rotor 26 and theelectromagnetic coil 34 at a rear side of the rotor 26. Although amagnetic flux circuit of the clutch 20 and details of the valve 32 aredescribed further below, in brief, magnetic flux from theelectromagnetic coil 34 can move (e.g., axially pivot) an armature 32-1,which in turn can move (e.g., concurrently pivot by pressing against) avalve element 32-2 (e.g., reed valve). The valve element 32-2 canselectively limit or prevent flow of the shear fluid along the fluidcircuit. In the illustrated embodiment, the valve element 32-2 coversand uncovers the outlet bore 44 to selectively control flow of the shearfluid out of the reservoir 28. In some embodiments, referred to as a“fail on” configuration, the valve element 32-2 can be mechanicallybiased to an open position by default, with energization of theelectromagnetic coil 34 causing the valve element 32-2 to move to aclosed position that limits or prevents shear fluid flow.

In the illustrated embodiment, the electromagnetic coil 34 is supportedon the shaft 22 by a coil bearing 48. More specifically, theelectromagnetic coil 34 can be supported at the driving end 22D of theshaft 22 outside of the housing 24, with the coil bearing 48 abuttingthe axial stop 22-3. The electromagnetic coil 34 is typicallyrotationally fixed by a tether or the like (not shown), with the coilbearing 48 allowing relative rotation between the non-rotatingelectromagnetic coil 34 and the rotatable shaft 22. Further details ofthe configuration of the electromagnetic coil 34 are discussed below.

FIG. 3 is a cross-sectional view of a portion of the clutch 20, shownonly above the axis A, illustrating a magnetic flux circuit that isrepresented schematically by a dashed line 50. The housing 24 includes afirst flux guide 24-4 and a second flux guide 24-5, each made of amagnetic flux-conductive material. Both the first and second flux guides24-4 and 24-5 are secured or embedded in the base 24-1 at a rear ordriving side of the rotor 26 in the illustrated embodiment. Moreover,each of the first and second flux guides 24-4 and 24-5 can protrude fromthe base 24-1 at opposite front and/or rear sides. The first and secondflux guides 24-4 and 24-5 each allow for the transmission of magneticflux through the housing 24, which is otherwise typically made of amaterial like aluminum that does not efficiently transmit magnetic flux.The second flux guide 24-5 can be configured as a hub located at aradially inner portion of the base 24-1, and can directly engage therear bearing 40. In this respect, the second flux guide 24-5 can be astructural portion of the housing 24 in addition to providing fluxtransmission functionality. The second flux guide 24-5 can alsoincorporate an axial stop to engage the rear bearing 40, and aradially-extending portion embedded in the base 24-1. The second fluxguide 24-5 can be located radially inward from and spaced apart from thefirst flux guide 24-4. In the illustrated embodiment, the first fluxguide 24-4 has opposite front and rear ends that are radially offsetrelative to each other in a stepwise manner.

The magnetic flux circuit of the clutch 20 that transmits magnetic fluxto facilitate actuation of the valve 32 has the following configurationin the illustrated embodiment. The flux circuit extends from theelectromagnetic coil 34 to the first flux guide 24-4 across a first airgap. Next, the flux circuit extends from the first flux guide 24-4 tothe armature 32-1 of the valve 32 across a second air gap. The fluxcircuit then extends from the armature 32-1 of the valve 32 to theconductive portion 26-1 of the rotor 26 across a third air gap. Thethird air gap can be located adjacent to the axially rearward surface26-4. Moreover, the third air gap can effectively close when thearmature 32-1 is actuated and drawn toward or against the rotor 26.Next, the flux circuit extends from the conductive portion 26-1 of therotor 26 to the second flux guide 24-5 across a fourth air gap. Thefourth air gap can be positioned adjacent to the cylindrical portion26-3. Lastly, the flux circuit extends from the second flux guide 24-5back to the electromagnetic coil 34 across a fifth air gap.

Some aspects of the flux circuit in the illustrated embodiment of theclutch 20 are as follows. Any or all of the first, second, fourth andfifth air gaps can be oriented radially. Radially-oriented air gaps in aflux circuit can be kept relatively small, with relatively tighttolerances, and help to promote efficient and consistent fluxtransmission. The third air gap can be oriented axially. As noted above,the third air gap can effectively close when the armature 32-1 isactuated and drawn toward or against the rotor 26. The first and secondair gaps can be located at approximately the same radial distance fromthe axis A, and magnetic flux passing from the armature 32-1 from theelectromagnetic coil 34 can cross both the first and second air gaps ina radially inward direction, due to the radial offset of the first fluxguide 24-4. The fifth air gap can be located outward of a radially innerperimeter of the electromagnetic coil 34, and the first air gap can belocated inside of a radially outer perimeter of the electromagnetic coil34. The entire flux circuit can be located outward of the bearings 40and 48, and, aside from passing through the conductive portion 26-1 ofthe rotor 26 can be arranged at a driving or rear side of the clutch 20.

FIG. 4 is a cross-sectional view of a portion of an electromagnetic coilassembly of the clutch 20, shown only above the axis A. The assemblyshown in FIG. 4 includes the electromagnetic coil 34 and the coilbearing 48.

The coil bearing 48 in the illustrated embodiment includes an outer race48-1, and inner race 48-2 and a plurality of rolling elements 48-3. Therolling elements 48-3 are arranged in between the outer and inner races48-1 and 48-2, and engage each of the races 48-1 and 48-2. In theillustrated embodiment, the coil bearing has a single row of ball-stylerolling elements 48-3, though other types of bearings (e.g., roller orneedle bearings) could be used in alternate embodiments.

As shown in FIG. 4, the electromagnetic coil 34 includes a coil housing34-1 and a winding or wire 34-2. The coil housing 34-1 is formed by awall 34-1W made of a magnetic flux conductive material that defines aninterior volume 34-3. In the illustrated embodiment, the coil housing34-1 is configured as a cup, with the wall 34-1W being open at one face(e.g., a forward face). The wall 34-1W of the coil housing 34-1 alsoforms a notch or step 34-4 that protrudes toward (or into) the interiorvolume 34-3. The coil bearing 48 is wholly or partially accommodatedwithin the step 34-4 at an exterior of the wall 34-1W of the coilhousing 34-1, that is, at an opposite side of the wall 34-1W from theinterior volume 34-3 and the turns of the winding 34-2. The presence ofthe step 34-4 establishes different portions (or sub-volumes) of theinterior volume 34-3, including a radially inner portion P_(I) and aradially outer portion P_(O). The inner and outer portions P_(I) andP_(O) of the interior volume 34-3 can be contiguous and adjoin eachother, and be open to each other without any barrier or obstructionbetween them. Moreover, the inner and outer portions P_(I) and P_(O) canbe arranged such that the interior volume 34-3 has an L-shape, whenviewed in cross-section. The outer portion P_(O) has an axial depth (orlength) D_(O), and the inner portion P_(I) has an axial depth (orlength) D_(I). The axial depth D_(O) is greater than the axial depthD_(I) in the illustrated embodiment. The outer portion P_(O) can bearranged radially outward of the outer race 48-1 of the coil bearing 48,while the inner portion P_(I) can extend radially inward of the outerrace 48-1 can overlap with the rolling elements 48-3 in the radialdirection (and optionally also the inner race 48-2 in furtherembodiments). In other words, the outer portion P_(O) can be arrangedradially outward of the coil bearing 48 while the inner portion P_(I)can be arranged axially side-by-side or overlapping with the coilbearing 48. In further embodiments, one or more additional steps couldbe provided in the wall 34-1W of the coil housing 34-1 (such as toaccommodate an additional coil bearing) and the interior volume 34-3could have corresponding additional portions.

The winding 34-2 forms a coil by making a plurality of turns that arelocated within the interior volume 34-3 of the coil housing 34-1. In theillustrated embodiment, the turns of the winding 34-2 span the inner andouter portions P_(I) and P_(O) of the interior volume 34-3, such thatthe coil formed by the winding 34-2 has a stepped shaped like the coilhousing 34-1. Moreover, magnetic flux is generated within both the innerand outer portions P_(I) and P_(O) of the interior volume 34-3 when theelectromagnetic coil 34 is energized. The configuration of the interiorvolume 34-3 allows the turns of the winding 34-2 to span at leastportions of the coil bearing 48 in both the radial and axial directions,for instance, the turns of the winding 34-2 can radially and axiallyspan the rolling elements 48-3 of the coil bearing 48.

The winding 34-2 can be potted within the coil housing 34-1 using asuitable potting material 34-5. Furthermore, a connection tower 34-6 canbe provided to facilitate making electrical connections between thewinding 34-2 and a power source (not shown). A tether or otherrotation-prevention device (not shown) can also be connected to theelectromagnetic coil 34.

The “stepped” configuration of the electromagnetic coil 34 facilitatesshortening the overall axial length of the clutch 20. The amount ofmagnetic flux that can be generated by the electromagnetic coil 34 isprimarily related to the number of turns of the winding 34-2 and anamount of current flowing in the winding 34-2. Two alternative designshighlight the challenges that these constraints present. From a purelymass/weight perspective, the most efficient design is to utilize a longand narrow coil where a diameter and thus a circumference of each turnof the winding is reduced or minimized Such a long, narrow coilconfiguration uses the least amount of material, reducing mass/weightand cost. Because the long, narrow electromagnetic coil is rotatablysupported on a shaft, such a long, narrow coil arrangement would tend toconsist of an electromagnetic coil and a coil bearing sittingside-by-side on a shaft. The close proximity of the long, narrowelectromagnetic coil to the shaft also allows the shaft to be used aspart of a magnetic flux circuit to operate a valve, if desired. On theother hand, from an axial length perspective, the most efficient clutchdesign is to have the electromagnetic coil directly over the coilbearing, putting the electromagnetic coil in a common axial footprintwith the coil bearing. However, because each turn of the common axialfootprint coil winding is now substantially longer in thecircumferential direction (due to a larger diameter outside the coilbearing), the material required to fabricate the electromagnetic coil issignificantly larger, adding cost and mass to the design. Furthermore,when the common axial footprint electromagnetic coil is used, the shaftcan no longer be easily used as part of the magnetic circuit without theaddition of other components (e.g., embedded flux guides). An embodimentof the clutch 20 in which the electromagnetic coil 34 has a steppedconfiguration is a combination of the two alternative designs (i.e., thelong, narrow coil and the common axial footprint designs), which allowsbenefits of each alternative design to be retained. The turns of thewinding 34-2 in the radially inner portion P_(I) that resideside-by-side with the coil bearing 48 allow for a relatively small turndiameter and close proximity to the shaft 22—although the illustratedembodiment of the clutch 20 does not use the shaft 22 for magneticconduction as part of the flux circuit, alternative embodiments of theclutch 20 could easily do so (see, for example, FIGS. 6 and 7).Additionally, the turns of the winding 34-2 in the radially outerportion P_(O) that reside above (i.e., radially outward from) the coilbearing 48 utilize a common axial footprint with the coil bearing 48,which would otherwise be unused axial length within the clutch 20. Thisconfiguration enables an axially small footprint of the electromagneticcoil 34 with a relatively small mass/weight penalty, while also allowingsuitable magnetic flux density to actuate the valve 32.

FIG. 5 is a cross-sectional perspective view of the rotor 26, thereservoir 28 and the valve 32, shown in isolation. In the illustratedembodiment, the valve 32 is carried by and secured to the rotor 26, andthe valve 32 is also positioned entirely on a rear (or driving) side ofthe rotor 26 opposite the reservoir 28.

As noted above, the valve 32 includes the armature 32-1 and the valveelement 32-2. Additionally, as shown in FIG. 5, the valve 32 includes aspring 32-3. In the illustrated embodiment, the armature 32-1 includesan annular body 32-1B with a central opening 32-1C. The central opening32-1C allows the annular body 32-1B to encircle the shaft 22 as well asa protruding portion of the first flux guide 24-4, the rear bearing 40,and/or other desired components (see FIGS. 2 and 3). In the illustratedembodiment, the central opening 32-1C has a slightly larger diameterthan the cylindrical surface 26-3 of the conductive portion 26-1 of therotor 26. The armature 32-1 forms a part of the flux circuit of theclutch 20, as explained above. The body 32-1B of the armature 32-1 canalso press against the valve element 32-2, which can be separatelyattached to the rotor 26, to move the valve element 32-2 so as to coverthe outlet bore 44.

In some embodiments, the configuration and operation of the valve 32 canbe similar to that described in commonly-assigned U.S. Pat. No.8,881,881, which utilizes a reed valve element. However, it should benoted that the particular configuration of the valve 32 disclosed hereinis provided merely by way of example and not limitation. Numerous othertypes of valve configurations can be utilized in alternativeembodiments, such as valves with translating or rotating elements, aswell as valves that selectively cover the return bore 46. Moreover,bimetal-controlled valve assemblies can be used in the furtherembodiments instead of an electromagnetically controlled valve assembly,as are well-known in the art.

The spring 32-3 (also called an anchor spring), which is a leaf springthat flexes in an axial direction along a radially projected line in theillustrated embodiment, is secured between the body 32-1B of thearmature 32-1 and the rotor 26 (e.g., at the non-conductive portion26-2). In this way, the spring 32-3 flexibly mounts the armature 32-1 tothe rotor 26, and can impart a biasing force to bias the armature 32-1,and the valve 32 as a whole, to an open position by default.

In the illustrated embodiment, the spring 32-3 is recessed relative tothe armature 32-1, which helps to reduce both an axial length and aradial dimension of the clutch 20. An axial depression 32-1D and anadjacent radially-extending cutout 32-1R can be provided in the body32-1B. In the illustrated embodiment, the cutout 32-1R has a generallyrectangular perimeter and extends inward from an outer diameter edge32-1E of the body 32-1B toward the central opening 32-1C, and extendsentirely through the body 32-1B between opposite front and rear sides(i.e., in the axial direction), while portion of the body 32-1B remainsintact between the cutout 32-1R and the central opening 32-1C. Thedepression 32-1D is located directly adjacent to and contiguous with thecutout 32-1R, and radially in between the cutout 32-1R and the centralopening 32-1C. The depression 32-1D has a depth that extends axiallythrough a portion of the body 32-1B from a rear side of the body 32-1B.The spring 32-3 extends into both the depression 32-1D and the cutout32-1R, and in the illustrated embodiment the spring 32-3 is generallywholly contained within the cutout 32-1R and the depression 32-1D(absent an extreme flexure condition that deflects a portion of thespring 32-3 outside the cutout 32-1R and/or the depression 32-1D). Oneend of the spring 32-3 can be attached to the rotor 26 (e.g., using asuitable fastener such as a screw) at a location inside the cutout 32-1Rand radially inward of the outer diameter edge 32-1E, and an oppositeend of the spring 32-3 can be attached to the body 32-1B of the armature32-1 within the depression 32-1D.

A stop opening 32-1S can also be provided in the body 32-1B of thearmature 32-1, located radially in between the central opening 32-1C andthe outer diameter edge 32-1E. The stop opening 32-1S can further belocated radially inward of, and angularly spaced from the outlet bore44. The stop opening 32-1S can cooperate with an armature stop 60 thatcan axially extend from the rotor 26. In the illustrated embodiment, thestop opening 32-1S is circular and extends entirely through the body32-1B between opposite front and rear sides. The stop 60 in theillustrated embodiment resembles a set screw or similar threadablyadjustable member, and is adjustable to establish an axial limit ofmovement of the armature 32-1 relative to the rotor 26. The stop 60 isarranged to align with and protrude into the stop opening 32-1S when thearmature 32-1 is actuated by energization of the electromagnetic coil34. Although only one stop opening 32-1S and stop 60 are visible in FIG.5, one or more additional, symmetrically arranged stop openings 32-1Sand stops 60 can further be provided.

FIG. 6 is a cross-sectional view of another embodiment of a viscousclutch 120, and FIG. 7 is a cross-sectional view a portion the clutch120. As shown in the embodiment of FIGS. 6 and 7, the clutch 120includes a shaft 122, a housing 124, a rotor 126, a reservoir 128, aworking chamber 130, a valve (or valve assembly) 132, and anelectromagnetic coil 134. An axis A is illustrated that defines arotational axis of the clutch 120. The general operation of the clutch120 is similar to that of the clutch 20 described above, with similarcomponents indicated by similar reference numbers increased by onehundred in FIGS. 6 and 7 and the accompanying text. Likewise, the clutch120 incorporates most of the same features and benefits of the clutch20. It should be noted that the illustrated embodiment of FIGS. 6 and 7is disclosed merely by way of example and not limitation, and numerousalternative embodiments are possible, some features of which aredescribed in the text that follows.

The shaft 122 is located at a center of the clutch 120, and extendsthrough an entire axial length of the clutch 120 including the housing124. As shown in the embodiment of FIG. 6, the shaft 122 includes afirst engagement feature 122-1 at a first or driving end 122D (alsocalled the rear end) and a second engagement feature 122-2 at anopposite second or front (or driven) end 122F. The shaft 122 in theillustrated embodiment is a “live” or driven input shaft, like the shaft22. In the illustrated embodiment, the first engagement feature 122-1 isconfigured as an internal (female) thread, and the second engagementfeatures 122-2 is configured as an external engagement feature, forinstance, like a hex head cap screw, wrench flats, or other suitableexternal tooling engagement structure. Outward of the internal firstengagement feature 122-1 can be a small axial stop 122-3 and acylindrical outer surface 122-4.

The housing 124 includes a base 124-1 and a cover 124-2 secured to eachother in a rotationally fixed manner. The base 124-1 can have a hub124-5 with an axially-extending and protruding ring 124-6, which canextend in a forward direction toward the rotor 126. The housing 124 isrotationally supported on the shaft 122, allowing the housing 124 toselectively rotate relative to the shaft 122 as a function of theoperational state of the clutch 120 (i.e., from approximately 0% toapproximately 100% of the rotational speed of the shaft 122). In thisrespect, where the shaft 122 (and rotor 126) accepts a torque input tothe clutch 120, the housing 124 can act as an output or output member ofthe clutch 120. Cooling fins can be provided on external surfaces of thehousing 124, similar to with the clutch 20 described above.

In the illustrated embodiment, the base 124-1 is supported on the shaft122 at the hub 124-5 by a rear bearing 140, and the cover 124-2 issupported on the shaft 122 by a front bearing 142. The rear and frontbearings 140 and 142 are located on opposite sides of the rotor 126, andare axially spaced apart from each other. The ring 124-6 of the hub124-5 can provide suitable axial space to support and engage the rearbearing 140 with the base 124-1 of the housing 124. The bearings 140 and142 are each single-row ball bearings in the illustrated embodiment,with the rear bearing 140 being larger than the front bearing 142, butthe bearings 140 and 142 could have other configurations in alternateembodiments.

The housing 124 can be made of aluminum or another suitable material ormaterials, and can be a die cast part. One or more flux guide insertscan pass through the housing 124, as discussed further below. However,it should be noted that an advantage of the clutch 120 is that only asmall number of flux guide inserts (e.g., a single flux guide) isneeded. An output device (not shown) such as a fan can be secured to thehousing 124 to accept a torque output from the clutch 120.

A spacer 125 can be provided adjacent to the rear bearing 140. In theillustrated embodiment, the spacer includes a front contact surface125-1 and an opposite rear contact surface 125-2. The front and rearcontact surfaces 125-1 and 125-2 can be parallel and axially-facingsurfaces, which can be radially offset from one another. The frontcontact surface can engage the rear bearing 140, and the rear contactsurface 125-2 can engage a shoulder of the shaft 122. In the illustratedembodiment, the spacer 125 has a hexagonal perimeter, in cross-section,to facilitate the radial offset of the front and rear contact surfaces125-1 and 125-2 as well as to accommodate a stepped configuration of theshaft 122. The spacer 125 can be made of a non-flux conductive material,or a flux conductive material (and therefore can optionally be part of amagnetic flux circuit, as discussed below).

The rotor 126 is positioned at least partially within the housing 124,and preferably entirely within the housing 124, and can have a disk-likeshape (accordingly, the rotor 126 can be called a rotor disk). The rotor126 is rotationally fixed to the shaft 122, and rotates at all timeswith the shaft 122. In this respect, where the shaft 122 accepts atorque input, the rotor 126 can function as the input or input member ofthe clutch 120.

In the illustrated embodiment, the rotor 126 has a conductive portion126-1 and a non-conductive portion 126-2, where “conductive” in thisinstance refers to magnetic flux conductivity. The conductive portion126-1 can be made of steel or another suitable magnetic flux-conductingmaterial (e.g., any ferromagnetic material), and the non-conductiveportion 126-2 can be made of aluminum or another suitable material thatdoes not readily conduct magnetic flux. The conductive portion 126-1 canbe configured as a radially inner hub of the rotor 126 that providesstructural support for the entire rotor 126, and the conductive portion126-1 can be directly secured to the shaft 122. The non-conductiveportion 126-2 can be located at a radially outer part of the rotor 126.The conductive portion 126-1 can be spaced from and separated from thereservoir 128. Moreover, in the illustrated embodiment the conductiveportion 126-1 abuts both the rear and front bearings 140 and 142. In theillustrated embodiment, the conductive portion 126-1 includes an axialoffset region that in turn includes a cylindrical portion with aninward-facing cylindrical surface 126-3, as well as an exposed axiallyrearward facing portion 126-4. Furthermore, in the illustratedembodiment, the conductive portion 126-1 forms the entirety of the rotor126 over a given radial extent (e.g., to or beyond the cylindricalsurface 126-3), and, in that respect, is a structural component that isdistinguished from a non-structural flux guide insert that is embeddedin or otherwise passes through surrounding structural material of arotor solely to conduct magnetic flux. The conductive portion 126-1 canalso extend radially outward from the shaft 122 over a significantdistance, making the conductive portion 126-1 more than a mere hub orinner sleeve. As shown in the illustrated embodiment, the conductiveportion 126-1 extends beyond the axial offset region and the cylindricalsurface 126-3, but is located radially inward of the working chamber130. Moreover, in the illustrated embodiment, the conductive portion126-1 extends from the shaft 122 to a location at or radially outwardfrom the ring 124-6 and/or the hub 124-5 of the base 124-1 of thehousing 124 as well as outward of the rear bearing 140. The conductiveand non-conductive portions 126-1 and 126-2 can overlap over a givenradial distance to facilitate a structural engagement 126-5 (e.g.,metallurgical or mechanical connection) between those portions. Thestructural engagement 126-5 can be an axially thickened portion of therotor 126, with an axial thickness that is larger than portions of therotor 126 both immediately radially inward and outward. The rotor 126can axially overlap a portion of the hub 124-5 of the base 124-1 of thehousing 124, such that a distal end of the axially-extending ring 124-6of the hub 124-5 of the base 124-1 of the housing 124 axially overlapsthe rotor 26 adjacent to the axial offset region at the cylindricalsurface 126-3. Such an arrangement helps to promote a compact overallaxial length of the clutch 120.

The reservoir 128 provides a storage volume to hold a supply of a shearfluid. In the illustrated embodiment, the reservoir 128 is provided onor within the rotor 126. A plate 128-1 of the reservoir 128 can beattached to and carried by the rotor 126 to form part of a boundary tohelp retain the shear fluid and to separate the reservoir 128 from otherportions of the clutch 120. The plate 128-1 can be located in aninterior of the clutch 120, and can be arranged at a front side of therotor 126 that faces the cover 124-2. All or part of the shear fluid canbe stored in the reservoir 128 when not needed for engagement of theclutch 120. In the illustrated embodiment, the reservoir 128 is carriedby the rotor 126, such that the reservoir 128 and shear fluid containedwithin both rotate with the rotor 126. An outlet bore 144 is providedalong the boundary of the reservoir 128 to allow the shear fluid to passto the working chamber 130 along a fluid circuit of the clutch 120. Inthe illustrated embodiment, the rotor 126 forms part of the boundary ofthe reservoir 128, and the outlet bore 144 passes through the rotor 126.More specifically, in the illustrated embodiment the outlet bore 144passes substantially axially through the non-conductive portion 126-2 ofthe rotor 126 at a location outward from the conductive portion 126-1.

The working chamber 130 is defined (and operatively positioned) betweenthe rotor 126 and the housing 124. The working chamber 130 can extend toboth sides of the rotor 126. As explained with respect to the clutch 20,selective introduction of the shear fluid (e.g., silicone oil) to theworking chamber 130 can engage the clutch 120 by creating a viscousshear coupling to transmit torque between the rotor 126 and the housing124, with the degree of torque transmission (and associated outputspeed) being variable as a function of the volume of shear fluid presentin the working chamber 130. Concentric annular ribs, grooves and/orother suitable structures can be provided on the rotor 126 and housing124 to increase surface area along the working chamber 130 and promote ashear coupling when the shear fluid is present in the working chamber130, as is known in the art. Moreover, openings (not shown) can beprovided in an outer diameter region of the rotor 126 to allow the shearfluid in the working chamber 130 to move between opposite side of therotor 126, in a manner well-known in the art.

The shear fluid is pumped from the working chamber 130 back to thereservoir 128 along a return bore 146, which is located in the housing124 in the illustrated embodiment. The pumping of the shear fluid intothe return bore 146 can occur continuously using a dam or baffle (notspecifically shown), as is known in the art. Such a dam can be locatedon the housing 124 adjacent to the inlet of the return bore 146. Thefluid circuit of the clutch 120 therefore extends from the reservoir 128to the working chamber 130 through the outlet bore 144, and then fromthe working chamber 130 back to the reservoir 128 through the returnbore 146.

The valve 132 selectively controls flow of the shear fluid between thereservoir 128 and the working chamber 130. The clutch 120 can beelectromagnetically controlled, meaning that selective energization ofthe electromagnetic coil 134 can control operation of the valve 132 inorder to control the volume of the shear fluid present in the workingchamber 130, and in turn the degree of engagement and torquetransmission between the input and output members (e.g., the rotor 126and the housing 124). In the illustrated embodiment, all moving parts ofthe valve 132 are contained within the housing 124, and the valve 132 ispositioned in between the rotor 126 and the electromagnetic coil 134 ata rear side of the rotor 126. Similar to the valve 32 of the clutch 20described above, magnetic flux from the electromagnetic coil 134 canmove (e.g., axially pivot) an armature 132-1, which in turn can move(e.g., concurrently pivot by pressing against) a valve element 132-2(e.g., reed valve). The armature 132-2 can be annular in shape, and canbe positioned around the ring 124-6 of the hub 124-5 of the housing 124and/or the rear bearing 140, which can help reduce the axial length ofthe clutch 120. Actuation of the valve element 132-2 by the armature132-2 can selectively limit or prevent flow of the shear fluid along thefluid circuit. In the illustrated embodiment, the valve element 132-2covers and uncovers the outlet bore 144 to selectively control flow ofthe shear fluid out of the reservoir 128. In some embodiments, referredto as a “fail on” configuration, the valve element 132-2 can bemechanically biased to an open position by default, with energization ofthe electromagnetic coil 134 causing the valve element 132-2 to move toa closed position that limits or prevents shear fluid flow. Furthermore,the valve 132 can have a configuration that is similar or identical tothe valve 32 described above and shown in FIG. 5, including having astop and a cutout for an anchor spring. In addition, because the valve132 and the armature 132-1 are carried by the rotor disk 126, and can beconfigured to be pulled toward the rotor disk 126 upon energization ofthe electromagnetic coil 134, there is no need for a stop to preventmotion relative to the housing 124, which can help reduce the axiallength of the clutch 120.

In the illustrated embodiment, the electromagnetic coil 134 is supportedon the shaft 122 by a coil bearing 148 that includes inner and outerraces 148-1 and 148-2 and rolling elements 148-3. More specifically, theelectromagnetic coil 134 can be supported at the driving end 122D of theshaft 122 outside of the housing 124, with the coil bearing 148 abuttingthe axial stop 122-3. The electromagnetic coil 134 is typicallyrotationally fixed by a tether or the like (not shown), with the coilbearing 148 allowing relative rotation between the non-rotatingelectromagnetic coil 134 and the rotatable shaft 122. Theelectromagnetic coil 134 and the coil bearing 148 can together provide acoil assembly similar to that described above with respect to FIG. 4.

FIG. 7 illustrates a magnetic flux circuit of the clutch 120 that isrepresented schematically by a dashed line 150. The housing 124 includesa flux guide 124-4 made of a magnetic flux-conductive material that issecured or embedded in the base 124-1 at a rear or driving side of therotor 126 in the illustrated embodiment. The flux guide 124-4 canprotrude from the base 124-1 at opposite front and/or rear sides. Theflux guide 124-4 can allow for the transmission of magnetic flux throughthe housing 124, which is otherwise typically made of a material likealuminum that does not efficiently transmit magnetic flux. In theillustrated embodiment, the flux guide 124-4 has opposite front and rearends that are radially offset relative to each other in a stepwisemanner, with a radially inwardly projecting ring.

The magnetic flux circuit of the clutch 120 that transmits magnetic fluxto facilitate actuation of the valve 132 has the following configurationin the illustrated embodiment. The flux circuit extends from theelectromagnetic coil 134 to the flux guide 124-4 across a first air gap.Next, the flux circuit extends from the flux guide 124-4 to the armature132-1 of the valve 132 across a second air gap. The flux circuit thenextends from the armature 132-1 of the valve 132 to the conductiveportion 126-1 of the rotor 126 across a third air gap. The third air gapcan be located adjacent to the axially rearward surface 126-4. Moreover,the third air gap can effectively close when the armature 132-1 isactuated and drawn toward or against the rotor 126. Next, the fluxcircuit extends from the conductive portion 126-1 of the rotor 126 tothe shaft 122. Lastly, the flux circuit extends from the shaft 122 backto the electromagnetic coil 134 across a fourth air gap.

Some aspects of the flux circuit in the illustrated embodiment of theclutch 120 are as follows. Any or all of the first, second, and fourthair gaps can be oriented radially. Radially-oriented air gaps in a fluxcircuit can be kept relatively small, with relatively tight tolerances,and help to promote efficient and consistent flux transmission. Thethird air gap can be oriented axially. As noted above, the third air gapcan effectively close when the armature 132-1 is actuated and drawntoward or against the rotor 126. The first air gap can be locatedoutward of a radially inner perimeter of the electromagnetic coil 134,and the fourth air gap can be located inside of a radially outerperimeter of the electromagnetic coil 134. The rear bearing 140 can belocated inside the flux circuit, while the front bearing 142 and thecoil bearing 148 can each be located outside the flux circuit. Asidefrom passing through the conductive portion 126-1 of the rotor 126, theflux circuit also can be arranged at a driving or rear side of theclutch 120.

The stepped configuration of the electromagnetic coil 134 can facilitateclose proximity to the shaft 122, thereby allowing flux transmissionbetween the electromagnetic coil 134 and the shaft 122 across arelatively small (fourth) radial air gap (see also FIG. 4 and theaccompanying description). Such a stepped electromagnetic coilconfiguration also avoids the need for additional flux guides, whichwould otherwise tend to undesirably increase manufacturing complexity,extend overall axial and/or radial dimensions of the clutch, andincrease mass.

Discussion of Possible Embodiments

The following are non-exclusive descriptions of possible embodiments ofthe present invention.

A viscous clutch can include an input member; an output member; aworking chamber defined between the input member and the output member;a reservoir to hold a supply of a shear fluid, the reservoir fluidicallyconnected to the working chamber by a fluid circuit; a valve, whereinthe valve controls a flow of the shear fluid between the reservoir andthe working chamber along the fluid circuit; a bearing; and anelectromagnetic coil supported by the bearing, wherein selectiveenergization of the electromagnetic coil actuates the valve, theelectromagnetic coil including a coil housing and a winding that formsmultiple turns within an interior volume of the coil housing, whereinthe coil housing has a stepped configuration to at least partiallyaccommodate the bearing within a first step.

The viscous clutch of the preceding paragraph can optionally include,additionally and/or alternatively, any one or more of the followingfeatures, configurations and/or additional components:

the interior volume of the coil housing can include a radially outervolume and a radially inner volume, wherein the radially outer volume islocated radially outward of the first step, and the radially innervolume overlaps the bearing;

the radially outer volume and the radially inner volume of the coilhousing can be adjoining and open to one another, and the turns of thewinding can span both the radially outer volume and the radially innervolume;

the bearing can include an outer race and rolling elements engaged withthe outer race, and the turns of the winding within the interior volumeof the coil housing can at least partially span the rolling elements inboth axial and radial directions;

the coil housing can be configured as a cup;

the winding can be potted within the coil housing;

a shaft, wherein the bearing supports the electromagnetic coil on theshaft;

the shaft can be rotationally fixed to the input member to rotate withthe input member at all times;

the input member can comprise a rotor disk, and the output member cancomprise a housing surrounding the rotor disk;

the shaft can extend entirely through the housing;

the electromagnetic coil can be positioned outside the housing;

the shaft can have an internal engagement feature at a first end and aninternal engagement feature at an opposite second end;

the shaft can have an internal engagement feature at a first end and anexternal engagement feature at an opposite second end;

the interior volume of the coil housing can be L-shaped incross-section;

the valve can include an armature having an annular body with a centralopening and a stop opening;

an armature stop can be arranged to align with and protrude into thestop opening in the annular body of the armature when the armature isactuated by energization of the electromagnetic coil;

the armature stop can comprise a threadably adjustable member configuredto adjust an axial limit of movement of the armature; and/or

the valve can include an armature having an annular body with aradially-extending cutout and an adjacent axial depression portion, thevalve including a leaf spring that flexibly mounts the armature to theinput member, wherein the leaf spring is wholly contained within theradially-extending cutout and the axial depression.

A method can include: winding a wire within an interior volume of anelectromagnetic coil housing to provide a coil, wherein the wire makesmultiple turns within the interior volume so as to span a radially outervolume and an adjoining radially inner volume, wherein the radiallyouter volume is located radially outward of a step formed by a wall ofthe electromagnetic coil housing; and engaging a bearing with theelectromagnetic coil housing at the step.

The method of the preceding paragraph can optionally include,additionally and/or alternatively, any one or more of the followingfeatures, configurations and/or additional steps:

supporting the electromagnetic coil housing with the bearing adjacent toa magnetic flux circuit that passes through anelectromagnetically-actuated valve of a viscous clutch.

An electromagnetic coil assembly for use with a clutch can include: abearing including an outer race, an inner race, and a plurality ofrolling elements positioned between the outer race and the inner race; awinding; and a coil housing defined by a wall supported on the outerrace of the bearing, wherein the coil housing further defines aninterior volume in which a plurality of turns of the winding arelocated, the turns of the winding located opposite the bearing acrossthe wall, wherein the interior volume includes a first portion having afirst axial depth and a second portion having a second axial depth, thefirst axial depth being greater than the second axial depth, wherein thefirst portion is located radially outward of the outer race of thebearing and the second portion extends radially inward of the outer raceof the bearing.

The electromagnetic coil assembly of the preceding paragraph canoptionally include, additionally and/or alternatively, any one or moreof the following features, configurations and/or additional components:

the coil housing can have a notch defined by the wall, wherein thebearing is contained within the notch in an axial direction;

the first and second portions of the interior volume can be contiguousand open to one another, and the turns of the winding can span both thefirst and second portions;

the turns of the winding can span both the first and second portions ofthe interior volume;

the turns of the winding within the interior of the coil housing can atleast partially span the rolling elements of the bearing in both axialand radial directions;

the coil housing can be configured as a cup;

the winding can be potted within the coil housing; and/or

the interior volume of the coil housing formed by the first and secondportions can be L-shaped in cross-section;

A viscous clutch can include: a shaft; a rotor disk rotationally affixedto the shaft to rotate at all times with the shaft, wherein the rotordisk includes a conductive portion made of a magnetic flux conductivematerial and another portion made of a different material; a housing; aworking chamber defined between the rotor disk and the housing, whereina torque coupling between the rotor disk and the housing is selectivelyprovided as a function of a volume of a shear fluid present in theworking chamber; a reservoir to hold a supply of the shear fluid, thereservoir fluidically connected to the working chamber by a fluidcircuit, wherein the reservoir is carried by the rotor disk; a valve,wherein the valve controls a flow of the shear fluid between thereservoir and the working chamber along the fluid circuit, the valveincluding an armature; an electromagnetic coil, wherein selectiveenergization of the coil controls actuation of the valve, theelectromagnetic coil located at an opposite side of the rotor disk fromthe reservoir; a first flux guide that passes through the housing; and asecond flux guide that passes through the housing and is located inwardof the first flux guide, wherein a magnetic flux circuit extends fromthe electromagnetic coil to the first flux guide, then to the armatureof the valve, then to the conductive portion of the rotor disk, then tothe second flux guide, and then back to the electromagnetic coil.

The viscous clutch of the preceding paragraph can optionally include,additionally and/or alternatively, any one or more of the followingfeatures, configurations and/or additional components:

the magnetic flux circuit crosses a first air gap between theelectromagnetic coil and the first flux guide, a second air gap betweenthe first flux guide and the armature of the valve, a third air gapbetween the armature of the valve and the conductive portion of therotor disk, a fourth air gap between the conductive portion of the rotorand the second flux guide, and a fifth air gap between the second fluxguide and the electromagnetic coil;

the first and fifth air gaps can each be oriented radially;

the fourth air gap can be oriented radially;

the second air gap can be oriented radially;

the third air gap can be oriented axially;

the second flux guide can form a hub of a base portion of the housingthat is supported on the shaft by a first housing bearing;

a cover portion of the housing can be supported on the shaft by a secondhousing bearing;

the first and second housing bearings can each be single row bearings;

the conductive portion of the rotor disk can form a hub that issupported on the shaft;

the conductive portion of the rotor disk can include a cylindricalsurface that faces the second flux guide across an air gap;

the shaft can extend entirely through the housing;

the electromagnetic coil can be positioned outside the housing;

the electromagnetic coil can be supported on the shaft by a coilbearing;

the electromagnetic coil can include: a coil housing; and a winding thatforms multiple turns within an interior volume of the coil housing,wherein the coil housing has a stepped configuration to at leastpartially accommodate the coil bearing within a first step at anexterior of the coil housing;

the interior volume of the coil housing can include a radially outervolume and a radially inner volume, wherein the radially outer volume islocated radially outward of the first step, and wherein the radiallyinner volume overlaps the coil bearing;

the radially outer volume and the radially inner volume of the coilhousing can be adjoining and open to one another, and the turns of thewinding can span both the radially outer volume and the radially innervolume;

the coil bearing can include an outer race and rolling elements engagedwith the outer race;

the turns of the winding within the interior volume of the coil housingcan at least partially span the rolling elements of the coil bearing inboth axial and radial directions;

the shaft can have an internal engagement feature at a first end and aninternal engagement feature at an opposite second end;

the shaft can have an internal engagement feature at a first end and anexternal engagement feature at an opposite second end;

the armature can have an annular body with a radially-extending cutoutand an adjacent axial depression portion, the valve including a leafspring that flexibly mounts the armature to the rotor disk;

the leaf spring can be wholly or partially contained within theradially-extending cutout and the axial depression;

the armature can have an annular body with a central opening and a stopopening;

an armature stop can be secured to the rotor disk, wherein the armaturestop is arranged to align with and protrude into the stop opening in theannular body of the armature when the armature is urged toward to therotor disk; and/or

the armature stop can include a threadably adjustable member configuredto adjust an axial limit of movement of the armature.

A viscous clutch can include a shaft; a rotor disk rotationally affixedto the shaft to rotate at all times with the shaft, wherein the rotordisk includes a conductive portion made of a magnetic flux conductivematerial and another portion made of a different material, wherein theconductive portion forms a hub of the rotor disk that contacts theshaft; a housing having a base and a cover, wherein the base includes ahub with an axially-extending ring; a working chamber defined betweenthe rotor disk and the housing, wherein a torque coupling between therotor disk and the housing is selectively provided as a function of avolume of a shear fluid present in the working chamber; a reservoir tohold a supply of the shear fluid, the reservoir fluidically connected tothe working chamber by a fluid circuit, wherein the reservoir is carriedby the rotor disk; a valve, wherein the valve controls a flow of theshear fluid between the reservoir and the working chamber along thefluid circuit, the valve including an armature, wherein the armature islocated radially outward of the axially-extending ring of the hub of thebase of the housing; an electromagnetic coil, wherein selectiveenergization of the coil controls actuation of the valve, theelectromagnetic coil located at an opposite side of the rotor disk fromthe reservoir; and a flux guide that passes through the housing, whereina magnetic flux circuit extends from the electromagnetic coil to theflux guide, then to the armature of the valve, then to the conductiveportion of the rotor disk, then to the shaft, and then back to theelectromagnetic coil.

The viscous clutch of the preceding paragraph can optionally include,additionally and/or alternatively, any one or more of the followingfeatures, configurations and/or additional components:

the magnetic flux circuit can cross a first air gap between theelectromagnetic coil and the flux guide, a second air gap between theflux guide and the armature of the valve, a third air gap between thearmature of the valve and the conductive portion of the rotor disk, afourth air gap between the shaft and the electromagnetic coil;

the first and fourth air gaps can each be oriented radially;

the second air gap can be oriented radially;

the third air gap can be oriented axially;

the base of the housing can be supported on the shaft by a first housingbearing engaged with the hub of the base, and the cover of the housingcan be supported on the shaft by a second housing bearing, and the firstand second housing bearings can each be single row bearings;

the conductive portion of the rotor disk can include a surface thatfaces the armature across an axial air gap;

the conductive portion of the rotor disk can be spaced from thereservoir;

the electromagnetic coil can be positioned outside the housing;

the electromagnetic coil can be supported on the shaft by a coilbearing;

the electromagnetic coil can include a coil housing; and a winding thatforms multiple turns within an interior volume of the coil housing,wherein the coil housing has a stepped configuration to at leastpartially accommodate the coil bearing within a first step at anexterior of the coil housing;

the interior volume of the coil housing can include a radially outervolume and a radially inner volume, the radially outer volume can belocated radially outward of the first step, and the radially innervolume can overlap the coil bearing;

the radially outer volume and the radially inner volume of the coilhousing can be adjoining and open to one another;

the turns of the winding can span both the radially outer volume and theradially inner volume;

the coil bearing can include an outer race and rolling elements engagedwith the outer race, and the turns of the winding within the interiorvolume of the coil housing can at least partially span the rollingelements in both axial and radial directions;

the shaft can have an internal engagement feature at a first end, andthe shaft can have an internal engagement feature at an opposite secondend;

the shaft can have an internal engagement feature at a first end, andthe shaft can have an external engagement feature at an opposite secondend;

the armature can have an annular body with a radially-extending cutoutand an adjacent axial depression portion, the valve can include a leafspring that flexibly mounts the armature to the rotor disk, and the leafspring can be wholly contained within the radially-extending cutout andthe axial depression;

the armature can have an annular body with a central opening and a stopopening, and the viscous clutch can further include an armature stopsecured to the rotor disk, and the armature stop can be arranged toalign with and protrude into the stop opening in the annular body of thearmature when the armature is urged toward to the rotor disk;

the armature stop can comprise a threadably adjustable member configuredto adjust an axial limit of movement of the armature;

the shaft can extend entirely through the housing;

the base of the housing can be supported on the shaft by a first housingbearing engaged with the hub of the base, the viscous clutch can furtherinclude a spacer abutting the first housing bearing, and the spacer canhave radially offset front and rear contact surfaces;

the base of the housing can be supported on the shaft by a first housingbearing, the cover of the housing can be supported on the shaft by asecond housing bearing, the first housing bearing can be located insidethe magnetic flux circuit, and the second housing bearing can be locatedoutside the magnetic flux circuit; and/or

the rotor disk can have an axial offset, and a distal end of theaxially-extending ring of the hub of the base of the housing axially canoverlap the rotor disk adjacent to the axial offset.

A method of operating a viscous clutch can include energizing anelectromagnetic coil to generate magnetic flux; passing the magneticflux from the electromagnetic coil to a flux guide in a housing of theviscous clutch across a first air gap; passing the magnetic flux fromthe first flux guide to an armature of a valve across a second air gap;passing the magnetic flux from the armature of a valve to a conductivehub portion of a rotor to across a third air gap; actuating the valve tocontrol flow of a shear fluid within the viscous clutch as a function ofmovement of the armature; passing the magnetic flux from the conductivehub portion of the rotor through a live shaft; and passing the magneticflux from the live shaft to the electromagnetic coil across a fourth airgap.

A method of operating a viscous clutch can include: energizing anelectromagnetic coil to generate magnetic flux; passing the magneticflux from the electromagnetic coil to a first flux guide in a housing ofthe viscous clutch across a first air gap; passing the magnetic fluxfrom the first flux guide to an armature of a valve across a second airgap; passing the magnetic flux from the armature to a conductive portionof the rotor across a third air gap; actuating the valve to control flowof a shear fluid within the viscous clutch as a function of movement ofthe armature; passing the magnetic flux from the conductive portion of arotor to a second flux guide in the housing across a fourth air gap; andpassing the magnetic flux from the second flux guide to theelectromagnetic coil across a fifth air gap.

A viscous clutch can include: a rotor disk; a housing; a working chamberdefined between the rotor disk and the housing, wherein a torquecoupling between the rotor disk and the housing is selectively providedas a function of a volume of a shear fluid present in the workingchamber; a reservoir to hold a supply of the shear fluid, the reservoirfluidically connected to the working chamber by a fluid circuit, whereinthe reservoir is carried by the rotor disk; a valve, wherein the valvecontrols a flow of the shear fluid between the reservoir and the workingchamber along the fluid circuit, the valve including an armature,wherein the armature has an annular body with a radially-extendingcutout and an adjacent axial depression portion, the valve including aleaf spring that flexibly mounts the armature to the rotor disk, whereinthe leaf spring is wholly contained within the radially-extending cutoutand the axial depression; and an electromagnetic coil, wherein selectiveenergization of the coil controls actuation of the valve.

A viscous clutch can include: a rotor disk; a housing; a working chamberdefined between the rotor disk and the housing, wherein a torquecoupling between the rotor disk and the housing is selectively providedas a function of a volume of a shear fluid present in the workingchamber; a reservoir to hold a supply of the shear fluid, the reservoirfluidically connected to the working chamber by a fluid circuit, whereinthe reservoir is carried by the rotor disk; a valve, wherein the valvecontrols a flow of the shear fluid between the reservoir and the workingchamber along the fluid circuit, the valve including an armature,wherein the armature has an annular body with a central opening and astop opening; an electromagnetic coil, wherein selective energization ofthe coil controls actuation of the valve; and an armature stop securedto the rotor disk, wherein the armature stop is arranged to align withand protrude into the stop opening in the annular body of the armaturewhen the armature is urged toward to the rotor disk.

The viscous clutch of the preceding paragraph can optionally include,additionally and/or alternatively, any one or more of the followingfeatures, configurations and/or additional components:

the armature stop can comprise a threadably adjustable member configuredto adjust an axial limit of movement of the armature.

SUMMATION

Any relative terms or terms of degree used herein, such as“substantially”, “essentially”, “generally”, “approximately” and thelike, should be interpreted in accordance with and subject to anyapplicable definitions or limits expressly stated herein. In allinstances, any relative terms or terms of degree used herein should beinterpreted to broadly encompass any relevant disclosed embodiments aswell as such ranges or variations as would be understood by a person ofordinary skill in the art in view of the entirety of the presentdisclosure, such as to encompass ordinary manufacturing tolerancevariations, incidental alignment variations, transient alignment orshape variations induced by thermal, rotational or vibrationaloperational conditions, and the like. Moreover, any relative terms orterms of degree used herein should be interpreted to encompass a rangethat expressly includes the designated quality, characteristic,parameter or value, without variation, as if no qualifying relative termor term of degree were utilized in the given disclosure or recitation.

Although the present invention has been described with reference topreferred embodiments, workers skilled in the art will recognize thatchanges may be made in form and detail without departing from the spiritand scope of the invention. For instance, while described as being forlight duty applications, a clutch of the present invention could bescaled up for medium or heavy duty applications as well. Moreover,features and configurations described with respect to one embodiment canbe incorporated into another embodiment, as desired.

1-25. (canceled)
 26. A viscous clutch comprising: a shaft; a rotor diskrotationally affixed to the shaft to rotate at all times with the shaft,wherein the rotor disk includes a conductive portion made of a magneticflux conductive material and another portion made of a differentmaterial, wherein the conductive portion forms a hub of the rotor diskthat contacts the shaft; a housing having a base and a cover, whereinthe base includes a hub with an axially-extending ring; a workingchamber defined between the rotor disk and the housing, wherein a torquecoupling between the rotor disk and the housing is selectively providedas a function of a volume of a shear fluid present in the workingchamber; a reservoir to hold a supply of the shear fluid, the reservoirfluidically connected to the working chamber by a fluid circuit, whereinthe reservoir is carried by the rotor disk; a valve, wherein the valvecontrols a flow of the shear fluid between the reservoir and the workingchamber along the fluid circuit, the valve including an armature,wherein the armature is located radially outward of theaxially-extending ring of the hub of the base of the housing; anelectromagnetic coil, wherein selective energization of the coilcontrols actuation of the valve, the electromagnetic coil located at anopposite side of the rotor disk from the reservoir; and a flux guidethat passes through the housing, wherein a magnetic flux circuit extendsfrom the electromagnetic coil to the flux guide, then to the armature ofthe valve, then to the conductive portion of the rotor disk, then to theshaft, and then back to the electromagnetic coil.
 27. The viscous clutchof claim 26, wherein the magnetic flux circuit crosses a first air gapbetween the electromagnetic coil and the flux guide, a second air gapbetween the flux guide and the armature of the valve, a third air gapbetween the armature of the valve and the conductive portion of therotor disk, a fourth air gap between the shaft and the electromagneticcoil.
 28. The viscous clutch of claim 27, wherein the first and fourthair gaps are each oriented radially.
 29. The viscous clutch of claim 28,wherein the second air gap is oriented radially.
 30. The viscous clutchof claim 29, wherein the third air gap is oriented axially.
 31. Theviscous clutch of claim 26, wherein the base of the housing is supportedon the shaft by a first housing bearing engaged with the hub of thebase, wherein the cover of the housing is supported on the shaft by asecond housing bearing, and wherein the first and second housingbearings are each single row bearings.
 32. The viscous clutch of claim26, wherein the conductive portion of the rotor disk includes a surfacethat faces the armature across an axial air gap.
 33. The viscous clutchof claim 26, wherein the conductive portion of the rotor disk is spacedfrom the reservoir.
 34. The viscous clutch of claim 26, wherein theelectromagnetic coil is positioned outside the housing.
 35. The viscousclutch of claim 26, wherein the electromagnetic coil is supported on theshaft by a coil bearing.
 36. The viscous clutch of claim 35, wherein theelectromagnetic coil comprises: a coil housing; and a winding that formsmultiple turns within an interior volume of the coil housing, whereinthe coil housing has a stepped configuration to at least partiallyaccommodate the coil bearing within a first step at an exterior of thecoil housing.
 37. The viscous clutch of claim 36, wherein the interiorvolume of the coil housing includes a radially outer volume and aradially inner volume, wherein the radially outer volume is locatedradially outward of the first step, and wherein the radially innervolume overlaps the coil bearing.
 38. The viscous clutch of claim 37,wherein the radially outer volume and the radially inner volume of thecoil housing are adjoining and open to one another, and wherein theturns of the winding span both the radially outer volume and theradially inner volume.
 39. The viscous clutch of claim 36, wherein thecoil bearing includes an outer race and rolling elements engaged withthe outer race, wherein the turns of the winding within the interiorvolume of the coil housing at least partially span the rolling elementsin both axial and radial directions.
 40. The viscous clutch of claim 26,wherein the shaft has an internal engagement feature at a first end, andwherein the shaft has an internal engagement feature at an oppositesecond end.
 41. The viscous clutch of claim 26, wherein the shaft has aninternal engagement feature at a first end, and wherein the shaft has anexternal engagement feature at an opposite second end.
 42. The viscousclutch of claim 26, wherein the armature has an annular body with aradially-extending cutout and an adjacent axial depression portion, thevalve including a leaf spring that flexibly mounts the armature to therotor disk, wherein the leaf spring is wholly contained within theradially-extending cutout and the axial depression.
 43. The viscousclutch of claim 26, wherein the armature has an annular body with acentral opening and a stop opening, the viscous clutch furthercomprising: an armature stop secured to the rotor disk, wherein thearmature stop is arranged to align with and protrude into the stopopening in the annular body of the armature when the armature is urgedtoward to the rotor disk.
 44. The viscous clutch of claim 43, whereinthe armature stop comprises a threadably adjustable member configured toadjust an axial limit of movement of the armature.
 45. The viscousclutch of claim 26, wherein the shaft extends entirely through thehousing.
 46. The viscous clutch of claim 26, wherein the base of thehousing is supported on the shaft by a first housing bearing engagedwith the hub of the base, the viscous clutch further comprising a spacerabutting the first housing bearing, wherein the spacer has radiallyoffset front and rear contact surfaces.
 47. The viscous clutch of claim26, wherein the base of the housing is supported on the shaft by a firsthousing bearing, wherein the cover of the housing is supported on theshaft by a second housing bearing, wherein the first housing bearing islocated inside the magnetic flux circuit, and wherein the second housingbearing is located outside the magnetic flux circuit.
 48. The viscousclutch of claim 26, wherein the rotor disk has an axial offset, andwherein a distal end of the axially-extending ring of the hub of thebase of the housing axially overlaps the rotor disk adjacent to theaxial offset.
 49. A method of operating a viscous clutch, the methodcomprising: energizing an electromagnetic coil to generate magneticflux; passing the magnetic flux from the electromagnetic coil to a fluxguide in a housing of the viscous clutch across a first air gap; passingthe magnetic flux from the first flux guide to an armature of a valveacross a second air gap; passing the magnetic flux from the armature ofa valve to a conductive hub portion of a rotor to across a third airgap; actuating the valve to control flow of a shear fluid within theviscous clutch as a function of movement of the armature; passing themagnetic flux from the conductive hub portion of the rotor through alive shaft; and passing the magnetic flux from the live shaft to theelectromagnetic coil across a fourth air gap.
 50. A viscous clutchcomprising: a rotor disk; a housing; a working chamber defined betweenthe rotor disk and the housing, wherein a torque coupling between therotor disk and the housing is selectively provided as a function of avolume of a shear fluid present in the working chamber; a reservoir tohold a supply of the shear fluid, the reservoir fluidically connected tothe working chamber by a fluid circuit, wherein the reservoir is carriedby the rotor disk; a valve, wherein the valve controls a flow of theshear fluid between the reservoir and the working chamber along thefluid circuit, the valve including an armature, wherein the armature hasan annular body with a radially-extending cutout and an adjacent axialdepression portion, the valve including a leaf spring that flexiblymounts the armature to the rotor disk, wherein the leaf spring is whollycontained within the radially-extending cutout and the axial depression;and an electromagnetic coil, wherein selective energization of the coilcontrols actuation of the valve.
 51. A viscous clutch comprising: arotor disk; a housing; a working chamber defined between the rotor diskand the housing, wherein a torque coupling between the rotor disk andthe housing is selectively provided as a function of a volume of a shearfluid present in the working chamber; a reservoir to hold a supply ofthe shear fluid, the reservoir fluidically connected to the workingchamber by a fluid circuit, wherein the reservoir is carried by therotor disk; a valve, wherein the valve controls a flow of the shearfluid between the reservoir and the working chamber along the fluidcircuit, the valve including an armature, wherein the armature has anannular body with a central opening and a stop opening; anelectromagnetic coil, wherein selective energization of the coilcontrols actuation of the valve; and an armature stop secured to therotor disk, wherein the armature stop is arranged to align with andprotrude into the stop opening in the annular body of the armature whenthe armature is urged toward to the rotor disk.
 52. The viscous clutchof claim 51, wherein the armature stop comprises a threadably adjustablemember configured to adjust an axial limit of movement of the armature.